The human eye functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens. When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. A typical surgical handpiece suitable for phacoemulsification procedures consists of an ultrasonically driven phacoemulsification handpiece, an attached hollow cutting needle surrounded by an irrigating sleeve, and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubing. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting needle. The flexible tubing supplies irrigation fluid to the surgical site and draws aspiration fluid from the eye through the handpiece assembly.
The operative part in a typical handpiece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting needle during phacoemulsification, and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece by flexible mountings. The handpiece body terminates in a reduced diameter portion or nosecone at the body's distal end. Typically, the nosecone is externally threaded to accept the hollow irrigation sleeve, which surrounds most of the length of the cutting needle. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting needle is adjusted so that its tip projects only a predetermined amount past the open end of the irrigating sleeve.
During the phacoemulsification procedure, the tip of the cutting needle and the end of the irrigation sleeve are inserted into the anterior capsule of the eye through a small incision in the outer tissue of the eye. The surgeon brings the tip of the cutting needle into contact with the lens of the eye, so that the vibrating tip fragments the lens. The resulting fragments are aspirated out of the eye through the interior bore of the cutting needle, along with irrigation solution provided to the eye during the procedure, and into a waste reservoir.
Throughout the procedure, irrigating fluid is pumped into the eye, passing between the irrigation sleeve and the cutting needle and exiting into the eye at the tip of the irrigation sleeve and/or from one or more ports, or openings, cut into the irrigation sleeve near its end. This irrigating fluid is critical, as it prevents the collapse of the eye during the removal of the emulsified lens. The irrigating fluid also protects the eye tissues from the heat generated by the vibrating of the ultrasonic cutting needle. Furthermore, the irrigating fluid suspends the fragments of the emulsified lens for aspiration from the eye.
A common complication during the phacoemulsification process arises from a blockage, or occlusion, of the aspirating needle. As the irrigation fluid and emulsified tissue is aspirated away from the interior of the eye through the hollow cutting needle, pieces of tissue that are larger than the diameter of the needle's bore may become clogged in the needle's tip. While the tip is clogged, vacuum pressure builds up within the tip. The resulting drop in pressure in the anterior chamber in the eye when the clog is removed is known as post-occlusion surge. This post-occlusion surge can, in some cases, cause a relatively large quantity of fluid and tissue to be aspirated out of the eye too quickly, potentially causing the eye to collapse and/or causing the lens capsule to be torn.
Various techniques have been attempted to reduce this surge, such as by venting the aspiration line or otherwise limiting the buildup of negative pressure in the aspiration system. However, there remains a need for improved phacoemulsification devices, including irrigation sleeves that reduce post-occlusion surge.